To perform a marine seismic acquisition in a survey area, it is common to use seismic sources (like “air guns”, “vibratory sources”, . . . ) and seismic sensors. The sensors are housed in cables, called streamers (or acoustic linear antennas or seismic cables). Several streamers are used together to form an array of thousands of sensors. Sources are towed by one or several vessels, and streamers are towed by one or several vessels. A same vessel can tow both sources and streamers (i.e. can tow one or several streamers and one or several seismic sources).
To collect the geophysical data in the marine environment, the seismic sources are activated to generate single pulses or continuous sweep of energy. The signals generated by each source travels through the different layers of the earth crust and the reflected signals are captured by the sensors (hydrophones) housed in the streamers. By processing the signals captured by the hydrophones, geophysicists are able to achieve an imaging of the different layers of the earth crust.
A seismic source should shoot at a shot point (also referred to as “shot point”), defined by its geographical coordinates (latitude/longitude and/or easting northing). When the vessel reaches this shot point, the seismic source is activated and produces an explosion. The set of shot points of all seismic sources is called “preplot”.
The marine seismic acquisition is controlled and monitored by a navigation system (also referred to as INS, for “Integrated Navigation System”), which is onboard each vessel. Each INS of a vessel allows computing position of sensors and seismic sources and driving the vessel along its acquisition path, according to a predetermined preplot, and to activate seismic sources to perform seismic acquisition at desired shot points of the preplot.
The navigation system also determines the moment of firing a source for each shot point, according to the positions of the various system components. This moment of firing is referred to as “shot time”.
To further increase the quality of seismic imaging, the seismic surveys can be performed in a well-known “multi-vessel” survey performed by a plurality of vessels.
In a multi-vessel survey, it is common to select a specific vessel among the plurality of vessels and to refer it to as a “master vessel”. This master vessel is a reference vessel and the reference of time of each other vessels thanks to its INS. Each other vessel is referred to as “slave vessel” and is synchronized on the reference of time of the master vessel. Such “reference/master vessel” is thus the reference to compute proper position to other vessels.
So that the shooting order is complied, the various vessels must be synchronized. The shooting order of the sources is defined by the preplot and must be performed as close as possible to the geographic coordinates of the shot points specified in the preplot.
More precisely, a seismic source of a vessel should perform its shot at a target location called “Bull's Eye” (also noted “BE”), this target location being computed during the survey from the reference vessel's positions. Each vessel, or more precisely the seismic source, has to match as possible this target location at the right time. In practice, a point of the reference vessel (or of any equipment associated with the reference vessel, e.g. a source) is used as reference point to calculate the ideal position of other vessels, i.e. for space synchronization of the vessels.
In the following description, it is assumed to describe a multi-vessel survey with one master vessel which is the reference vessel and at least one other vessels of the fleet, also referred to as “slave vessels”.
In such a configuration, a Bull's Eye of a slave vessel represents the ideal position where said slave vessel (or seismic source of a slave vessel) should be to perform its shot, said Bull's Eye being defined by a circular target having:                a center, called Bull's Eye Position, which depends on the projection of the master vessel's reference point on a sail line of the slave vessel, a sail line being the way to follow by a vessel to perform its preplot. In the particular case where the slave vessels are supposed to be aligned with the master vessel, the center of the “Bull's Eye” is coincident with the projection of the master vessel's reference point on the sail line of the slave vessel. In the particular case where the slave vessels are not supposed to be aligned, there is a predetermined offset, along the sail line of the slave vessels, between the center of the slave vessel's “Bull's Eye” and the projection of the master vessel's reference point on the sail line of the slave vessels; and        a radius of tolerance which can be determined by contract requirements (e.g. 10 m).        
A reference point of the slave vessel (i.e. a seismic source) defined in advance, must be located in the “Bull's Eye” to ensure proper synchronization of said slave vessel.
A master vessel “Time to shot” (or “Shot Time T0” or “shooting time” or “master's vessel predictions”) is usually computed from:                master path and its shot points locations,        a distance between a shot point of the preplot and a master shot predict point location (master shot predict point is usually the air guns (sources) but can also be any other point owned by the master vessel),        master vessel speed along the path.        
Usually, the “time to shot” for slave vessels is computed from a master vessel shooting time to ensure good shot scheduling. In other way, slave vessels can compute their own time to shot based on the master's vessel predictions.
To have a proper coverage, during a multi-vessel operation, each vessel has to be properly aligned to shot and record data, at the best location as possible. For that, it is necessary to control the vessels speed.
Usually, the master vessel computes speed order depending of its own sensors, and a slave vessel computes its speed relative from master's speed and master's position.
Thus, in a multi vessel configuration, there is a strong link with the reference vessel. This is a main disadvantage of this configuration. Indeed, if radio links are broken, slave vessels have no more the relative position of the master vessel. If the reference vessel has any issue and presents errors in its shots, the relative position of slave vessels will be “wrong” as they are based on a “wrong” master vessel's position. Moreover, if the master vessel has any issue and has to stop the line, slave vessels have to stop production even if they are able to continue.
Moreover, most of the other known systems are restrictive, as they work with relative positions. Indeed, in that case, all vessels must have the same kind of path (symmetric path, parallel path or the same path with an offset).